The invention is directed to a method and apparatus for determining the cumulative load stress on hoisting equipment, under consideration of various types of load stress.
Hoisting installations are described and classified in such a way, pursuant to the agreements and guidelines of the Federation Europeanne de la Manunetion (European Federation of Conveyor Technology, hereinafter, the FEM) that, for an intended application, a known life expectancy for the equipment, expressed in years, may be expected. To this end, the hoisting installations are divided into six groups, according to certain operating conditions known to effect equipment life expectancy. These operating conditions are
1. The mean running time per day, and PA1 2. The cube of the mean value of the cumulative load.
In choosing a hoisting installation, values for both these magnitudes are assumed. The point here is to know as accurately as possible both of these values for the specific case of application. In a known comparable individual case of a hoisting installation, the running time may be determined somewhat reliably by timing with the aid of a service-hour meter. The cube of the mean value of the cumulative load, however, which is a time-dependent as well as a load-dependent magnitude, may be approximated only by gross estimates of the prevailing number of cases of application.
The FEM (Section IX) has issued the following formula for calculating the cube of the mean value of the stress from the cumulative load: ##EQU1## wherein: ##EQU2##
In order to economize on the expenditures involved in the calculation of K, it is customary to divide the cumulative load into a limited number of load categories for which the relative running period for each category is then determined. This measuring process is, however, technically very expensive and not suitable for continuous monitoring of the hoisting installation during operation.
The classification of the hoisting equipment, in view of the cube of the mean value of the cumulative load, is based on the premise that the life time of the individual building elements depends on the value of the load cubed. Accordingly, a correlation is established between the relative wear on a hoisting installation and the pair of values consisting of total running time and cube of the mean value of the cumulative load. The correlation may also serve to indicate proper times for maintenance and prevantive maintenance of the hoisting equipment. Since, however, all basic data rest on assumptions, the calculated maintenance periods are unreliable estimates.
It is therefore the objective of the invention to provide a reliable monitoring and evaluation method for hoisting equipment. This objective is achieved by measuring the load and the running time, and, during the running time, to weigh the measured results according to their contribution to the wear on the hoisting equipment. The measured data is summed to a value characterizing the stress on the hoisting equipment, and information relating to stress conditions is indicated to an operator when a predetermined permissible stress point is reached. This information takes into account the factual stress and does not rely as previously on estimates.
The relative load is advantageously measured in equal chronological intervals or measuring cycles. The chronological intervals may, for example, be one tenth of a second and include intervals during which peak values of stress occur such as, for example, vibrations of a load at the point of initial lifting.
A value for the relative load p=(.beta.+.gamma.) may be generated for use in a circuit according to the invention in terms of the output signal, (as either an analogue or digital value), of a load-measuring apparatus (known per se) operatively connected to the hoisting equipment, co-related to an output signal of the same apparatus corresponding to the carrying capacity of the hoisting equipment. Such an apparatus measures all of the components of the total load, i.e., live load, weight of load-lifting means and braking means, as well as the active weight of the carrying means.
By virtue of the relative load p being measured in equal chronological intervals, the relative running time t is equal to the sum of the individual measured values of p so that the sum of all values p.sub.i.sup.3 (i=1 to n, n=number of measuring cycles) is a characteristic value for the stress on the hoisting equipment. The permissible value for ##EQU3## is accurately definable for a hoisting installation designed according to FEM rules. With these values it is possible to determine accurately the points of time for maintenance corresponding to the stress. Also accurately determinable is the point of time at which the hoisting installation has reached its predetermined life expectancy.
The determination of the cumulative load stress values, according to the method of the invention, allows for the calculation of intermediary results in a simple manner, said results characterizing the unit stress and yielding the determined stress value. An advantageous feature of the invention is that is generates information relating to the stress condition on reaching the permissible amount of stress, thereby not only indicating that a predetermined stress value has been reached, but additionally containing the reached stress value and/or the running time and/or the number of switch actions and/or the number of measuring cycles. The additional indication of the running time makes it possible to determine, in which running-time category the hoisting equipment was operated. The number of switching actions shows under how much stress the drive was put during peak performances. The cube of the mean stress value may be calcuated from the stress value and the number of measuring cycles.
In some cases, and under conditions where the particular effect on wear was not recognized and may not be estimated, it is to be expected that the relative wear is greater than would be expected from the value for stress. In order to ascertain that the hoisting equipment is monitored in such cases as well, it is appropriate to monitor particularly wear-prone parts. This may be done in the conventional manner by monitoring the effects of the stress with the aid of boundary-value indicators for wear of the brake lining, the slowing-down period during the braking process, number of rotations of the drum, cable elongation, motor temperature, and oil level. In combining these boundary-value indicators with the determination of the stress values and the given condition information, a relationship between the wear of certain building groups and the stress value may be determined, even under special operating conditions, thereby making it subsequently possible to establish, in a simple manner, with the help of the stress value, the optimal point of time for preventive maintenance.
An apparatus for applying the method of determining the stress on hoisting equipment under consideration of the different loads is characterized by electrically connected electrical components including a timing generator, gate switch, analogue-to-digital converter, adder, memory, boundary-value comparator, and boundary-value indicator, as well as pulse-timing link connected between the analogue-to-digital converter and the memory. An analogue root-determining device may be connected to the analogue-to-digital converter. A digital root-determining device may be arrnged between the analogue-to-digital converter and the pulse-timing link as well as the adder. An impulse counter and a condition indicator may be series connected to the gate switch. A condition indicator may also be connected to the memory.
The hoisting installation is, according to the invention, equipped with an apparatus which forms the value for the stress according to the algorithm determined by introducing a constant measuring interval.
For a better understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of preferred embodiments of the invention and to the accompanying drawings.